This invention relates to the disproportionation of alkylaromatic feedstreams and more particularly to the disproportionation of toluene containing feedstocks employing mordenite catalysts of low aluminum content.
The disproportionation of toluene involves a well known transalkylation reaction in which toluene is converted to benzene and xylene in accordance with the following reaction which is mildly exothermic. 
Mordenite is one of a number of catalysts commonly employed in the transalkylation of alkylaromatic compounds. Mordenite is a crystalline aluminosilicate zeolite having a network of silicon and aluminum atoms interlinked in its crystalline structure through oxygen atoms. For a general description of mordenite catalysts, reference is made to Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition, 1981,under the heading xe2x80x9cMolecular Sievesxe2x80x9d, Vol. 15, pages 638-643. Mordenite, as found in nature or as synthesized, typically has a relatively low silica to alumina mole ratio of about 10 or less. Such conventionally structured mordenite catalysts are commonly employed in the disproportionation of toluene. However, mordenite catalysts having substantially lower alumina content are also employed in the disproportionation of toluene.
The aluminum deficient mordenite catalysts have a silica/alumina ratio greater than 10 and may sometimes range up to about 100. Such low alumina mordenites may be prepared by direct synthesis as disclosed, for example, in U.S. Pat. No. 3,436,174 to Sand or by acid extraction of a more conventionally prepared mordenite as disclosed in U.S. Pat. No. 3,480,539 to Voorhies et al. U.S. Pat. No. 3,780,122 to Pollitzer discloses the transalkylation of toluene using a mordenite zeolite having a silica/alumina ratio greater than 10 which is obtained by acid extraction of a mordenite zeolite having a silica/alumina ratio of less than 10.
The disproportionation of toluene feedstocks may be carried out at temperatures ranging from about 200xc2x0 C. to about 600xc2x0 C. or above and at pressures ranging from atmospheric to perhaps 100 atmospheres or above. However, the catalyst itself may impose constraints on the reaction temperatures in terms of catalyst activity and aging characteristics. In general, the prior art indicates that while relatively high temperatures can be employed for the high aluminum mordenites (low silica to alumina ratios) somewhat lower temperatures should be employed for the low alumina mordenites. Thus, where mordenite catalysts having high silica/alumina ratios have been employed in the transalkylation of alkylaromatics, it has been the practice to operate toward the lower end of the temperature range. It is also a common practice in this case to promote the catalyst with a catalytically active metallic content, as disclosed, for example, in U.S. Pat. No. 3,476,821 to Brandenburg. Metal promoters are said to substantially increase activity and catalyt life.
It is conventional practice to supply hydrogen along with toluene to the reaction zone. While the disproportionation reaction (1) is net of hydrogen, the use of a hydrogen co-feed is generally considered to prolong the useful life of the catalyst, as disclosed, for example, in the above patent to Brandenburg. The amount of hydrogen supplied, which can be measured in terms of the hydrogen/toluene mole ratio or in terms of a standard liter of hydrogen per liter of feedstock, is generally shown in the prior art to increase as temperature increases. Normally, the hydrocarbon feedstock supplied to the toluene disproportionation reaction zone is of extremely high purity. Typically, feedstocks having a toluene content of 90-99 wt. % are supplied to the reaction zone. Usually it is considered desirable to maintain the toluene content in excess of 99 wt. % (less than 1% impurities) in order to avoid unacceptably rapid catalyst deactivation. Thus the atmosphere within the reaction zone is, in addition to the hydrogen co-feed, toluene reactant and benzene and xylene products.
In accordance with the present invention there is provided a novel process for the disproportionation of toluene over a mordenite catalyst under conditions in which the feedstock has a lower toluene content than normally encountered and contains, in addition, a significant content of non-aromatic hydrocarbons, typically containing from 6-8 carbon atoms. In carrying out the invention, a toluene feedstock and a hydrogen co-feed are supplied to a reaction zone containing a mordenite-type disproportionation catalyst. The toluene feedstock has a toluene content within the range of 80-90 wt. % toluene and a C6-C8 non-aromatic content within the range of 10-20 wt. %. The reaction zone is operated under temperature and pressure conditions effective to cause a disproportionation of the toluene to benzene and xylene with the concomitant cracking of the non-aromatic hydrocarbons to convert a predominant portion of the non-aromatic hydrocarbon content to lower molecular weight hydrocarbons and produce a lower boiling fraction in the LPG boiling range. A product stream comprising toluene, benzene, xylene and C2-C4 aliphatic hydrocarbons is recovered from the reaction zone.
In a preferred embodiment of the invention, the mordenite catalyst is promoted with a metal which is effective to enhance the hydrogenation activity of the catalyst. In one application of the invention, the mordenite catalyst is promoted with nickel. In another, the mordenite catalyst is promoted with palladium or platinum. Preferred operating conditions for the invention involve a reaction zone temperature within the range of 300xc2x0-500xc2x0 C. and an average pressure within the reaction zone within the range of 20-60 bars. The feedstock is supplied to the reaction zone to provide a liquid hourly space velocity (LHSV) within the range of 0.5 hoursxe2x88x921-4.0 hoursxe2x88x921.
In a further aspect of the invention, there is provided a toluene disproportion process in which the reaction severity conditions in the reaction zone are adjusted in order to accommodate the toluene content of the feedstock. In carrying out this aspect of the invention, there is supplied to the reaction zone a first toluene continuing feedstock having a first relatively high toluene content. Hydrogen is also supplied to the reaction zone. The reaction zone is operated under reaction severity conditions of space velocity, temperature and pressure effective to cause disproportionation of the toluene in the feedstock to benzene and xylene and a product stream containing toluene, benzene and xylene as recovered from the reaction zone. Thereafter, a second toluene-containing feedstock is supplied to the reaction zone. The second toluene-containing feedstock has a second toluene content, which is lower than the toluene content of the first highly pure feedstock, and also has a C6-C8 non-aromatic content which is greater than any content of C6-C8 non-aromatic hydrocarbons in the first feedstock. Concomitantly with the supply of the second feedstock, the reaction zone is operated under reaction severity conditions of space velocity, temperature, and pressure which are more severe than the reaction severity conditions of the reaction zone when supplied with the highly pure toluene feedstock. The more severe reaction conditions are effective to cause disproportionation of the toluene to benzene and xylene with concomitant cracking of the non-aromatic hydrocarbons to convert the predominant portion of the non-aromatic hydrocarbon content to lower molecular weight hydrocarbons to produce a lower boiling fraction in the LPG boiling range. The product stream recovered from the reaction zone contains toluene, benzene, xylene and C2-C4 hydrocarbons. Preferably in this aspect of the invention, the reaction zone under the more severity reaction conditions involves a lower space velocity and preferably also a higher pressure than operation of the reaction zone during feed of the highly pure toluene feedstock.
Preferably the second feedstock has a toluene content which is lower than the toluene content of the first feedstock by an incremental amount of at least an 8 wt. %. Further, the second feedstock contains C6-C8 non-aromatic hydrocarbons in an amount which is incrementally at least 8 wt. % greater than the content of any C6-C8 non-aromatic hydrocarbons in the first feedstream. In a specific application of the invention the toluene content of the first feedstock has a toluene purity greater than 90 wt. % and the toluene content of the second feedstock is within the range of 80-90 wt. % toluene and has a C6-C8 non-aromatic content he range of 10-20 wt. %.